This invention relates to the radiating elements used in radio frequency antenna arrays such as are found, for example, in certain radar equipment and more especially it relates to very wide frequency bandwidth operation of such antenna arrays.
Electromagnetic energy is radiated from and is received by specially designed antenna structures which can exist in many topological forms. Very common and simple antenna structures are seen in applications to automobile broadcast radio reception and domestic television reception. More complicated antenna structures can be seen in radar equipment used to detect distant moving targets for both military and civil purposes.
The most complex radar antennas are examples of a class of antenna arrays, employing a plurality of individual small antenna elements which are interconnected in ways designed to enable, for example, electronic steering of the radiated beams of electromagnetic energy in space, without physical movement of the whole array.
Individual antenna elements forming an array can be, for example, simple dipoles which are well known. Such elements are referred to as fundamental elements and usually have the smallest possible dimensions for a given frequency of the radiated energy (FIG. 1). The dipole arms 1a and 1b are usually each one quarter-wavelength long at the frequency of operation and are spaced one quarter wavelength x above a metallic ground plane 2 to give radiation in the desired direction z. Transmission line 3 supplies energy to the dipole arms 1a and 1b. The ratio of length 1 to diameter d is usually&gt;10, which gives satisfactory performance over a narrow frequency band of a few percent with respect to the centre frequency of the band. The direction of the electric field vector is indicated by the arrow E.
Antenna Arrays can be made using a plurality of such elements, distributed uniformly or non-uniformly over a prescribed surface area, and chosen to provide the desired antenna radiation characteristics. The surface may be planar or curved in more than one plane and the perimeter may be of any shape, though it is commonly circular, or rectangular, or simply a straight line, which is the degenerate case for a rectangular aperture when one side of the rectangle has zero dimension.
FIG. 2 shows a rectangular array of M.times.N dipole elements 5 located over a metallic ground plane 6. Antenna elements in the array are spaced from each other by locating them on the nodal points of a geometrical lattice 4, which might be for example either rectangular (as shown) or triangular in nature. Spacing of the elements 5 from each other s, p, and d cannot exceed certain maximum fractions of the wavelength of the radiated electromagnetic energy if undesirable features in the array polar pattern are to be avoided. If this maximum element spacing is exceeded, in an attempt to minimise the number of elements in the array, then "grating lobes" are generated in the polar pattern of the radiated energy from the array. Grating lobes are replicas of the main (fundamental) lobe of the pattern but they are in different spatial directions from it.
In radar applications it is not possible to distinguish between targets detected in the main beam and in the grating lobe beams which results in ambiguities. A target detected in a grating lobe beam will be processed as if it had been received in the main beam and will be assigned a completely erroneous spatial direction by the radar signal processor. In radar and in other applications, such as broadcasting and communications services, grating lobes carry some of the energy to unwanted spatial regions and so reduce the operating efficiency of the system.
It is usually possible, for most narrow frequency bandwidth applications, to accept the array element spacing limitation. If the main beam of the radiated pattern is not to be electronically scanned the spacing d in FIG. 2 can be up to one half-wavelength at the operating frequency. If the beam is to be electronically scanned the spacing must be reduced as the maximum scan angle increases, down to a minimum of one half-wavelength for a scan of ninety degrees from the normal to the array surface.
However, there are occasions when it is necessary to transmit and receive electromagnetic energy over a wide frequency range, for example in frequency agile radars which operate at one or more frequencies distributed over a prescribed wide frequency range. Frequency agility can allow the radar or tactical communications system to continue to operate when interference, of whatever nature, overwhelms reception on any one frequency. Agility has other advantages in target detection and signal processing that are commonly exploited in radar equipment, particularly those applied to military functions.
It is usually desirable in such frequency agile military applications to operate over as wide a frequency band as possible; at least an octave. This requires that the individual elements of the array are capable of operating over the chosen frequency range and that their separations from each other meet the maximum spacing criterion already described, at all operating frequencies. Clearly this is not possible with conventional antenna elements such as single linear dipoles, even though there are established designs for wide-band dipoles which permit operation over a band-width of about 30% with respect to the mean frequency of the band. For example, a broad band half wave dipole is described in IEEE Transactions on Antennas and Propagation, Vol AP-32, No. 4, April 1984 pp 410-412 by M. C. Bailey and describes a bow-tie shaped dipole, which has a length equal to 0.32 of the mean wave-length of the band of operation, and has been shown to have acceptable performance over a 33% band-width, centred around 600 MHz, determined on the criterion that the input Voltage Standing Wave Ratio (VSWR) shall not exceed 2.0.
Even if it was possible to make a dipole capable of radiating over an octave change in frequency, it could not satisfy the separation condition necessary to ensure grating lobe free radiation over the octave range, from an array formed by a plurality of such dipoles. The length of the dipole would be between one half-wavelength at the lowest frequency and one half-wavelength at the highest frequency, and so the separation between dipoles in the array must exceed a half-wavelength at the highest frequency if physical interference between dipoles is to be avoided. Mathematical modelling of the bow-tie dipole described in the previously mentioned article in IEEE Transactions on Antennas and Propagation, using the proven analysis software Numerical Electromagnetic Code (NEC), has shown that it cannot be designed to operate over an octave frequency range.
The elements used in an array antenna need not be single dipoles. A Log-Periodic Dipole Array (LPDA) as shown in FIG. 3, in which a series of half-wavelength dipoles arranged in a coplanar and parallel configuration on a parallel wire transmission line 7, may be used as a very broadband element. The five element LPDA shown in FIG. 3 is representative of the LPDA class of antennas. The number of dipole elements used in the LPDA depends on the required performance characteristics. The lengths and spacing of the dipoles in the LPDA increase logarithmically in proportion to their distance from a fixed co-ordinate reference point 8. Energy is fed to the LPDA from the feed point 9 which is close to the dipole 10, in a direction towards the reference point 8.
The first and last dipoles 10 and 11 respectively are chosen to suit the frequency band of interest which can be several octaves or even a decade in extent. Dipole 10 will have dimensions chosen to make it radiate correctly at the high frequency end of the band. A metallic ground plane 12 is located approximately one quarter-wavelength at the lowest operating frequency from dipole 11 to provide unidirectional radiation which may be desirable in applications of the invention to radar for example, where energy radiated in the backward direction may have adverse effects on the operation of the radar. Transmission line 7 is short circuited by metallic ground-plane 12 where it intersects it at point A. Such LPDA's are well known, for example UK patent no. 884889 describes such an LPDA, and are in wide use. The direction of the electric field vector radiated or received by the LPDA, known as the polarisation of the wave, is shown by the arrow E. It lies in the common plane of the dipoles (horizontal as drawn) because the dipole excitation currents all lie in that plane.
A planar array antenna could comprise a plurality of LPDA elements arranged with the planes containing their individual sets of dipoles being normal to the planar array. FIG. 4 shows elements 14-18 in the array, located on the nodal points of rectangular lattice 19.
A planar array so formed has the advantage that the side-lobes of the pattern at wide angles from its normal direction are reduced, compared with the side-lobes from a corresponding array of single dipole elements, since the beamwidth of the LPDA element is narrower than that of the dipole element. However, the same element spacing criterion which applies to the array of the dipole elements to eliminate grating lobes applies to the array of LPDA elements, but the grating lobe magnitudes will be reduced by the narrow beam pattern of the LPDA element.
The LPDA overcomes the frequency bandwidth limitations of the single dipole element but, just as with the single wide bandwidth dipole, it fails to meet the spacing criterion necessary to suppress grating lobes generated by the planar array. For example, LPDA's 14 and 15 in FIG. 4 cannot be positioned closer in the array than the longest dipole element, 11 in FIG. 3, will allow. When this is done the high frequency elements, 20 in LPDA's 14 and 15 will be separated from each other by more than one half-wavelength at the high frequency; in fact by one wavelength if the LPDA is designed to operate over an octave, and grating lobes will be formed at the higher frequencies in the operating band.